Method for vacuum pressure forming reinforced plastic articles

ABSTRACT

A method for thermoforming a reinforced article, comprising: providing a reinforced plastic sheet comprising at least one thermoplastic material and reinforcement nanoparticles dispersed within the at least one thermoplastic material, the reinforcement particles comprising less than 15% of a total volume of the plastic sheet, at least 50% of the reinforcement particles having a thickness of less than about 20 layers, and at least 99% of the reinforcement particles having a thickness of less than about 30 layers; preheating said plastic sheet; communicating said preheated plastic sheet to a first mold assembly having a first mold cavity defined by mold surfaces, the mold surfaces corresponding to a configuration of the article to be molded, an amount of the plastic sheet communicated to the first mold assembly being sufficient to form a skin of the article; applying a vacuum to one side of the first mold assembly while concurrently applying pressurized gas to an opposing side of the first mold assembly so as to force said heated plastic sheet into conformity with the mold surfaces; cooling the conformed plastic sheet; transferring the conformed plastic sheet to a second mold assembly; introducing to the conformed plastic sheet a reinforced plastic melt made from material identical or different from that of the plastic sheet, said plastic melt having a blowing agent to achieve volume expansion and the production of a cellular reticulate structure; cooling said plastic melt to form a solidified plastic member adhered to said conformed plastic sheet, said conformed plastic sheet and said adhered solidified plastic member together comprising said article; and removing said article from said second mold assembly.

This patent application claims priority from U.S. ProvisionalApplication No. 60/113,064, filed Dec. 21, 1998.

FIELD OF THE INVENTION

The present invention relates to vacuum forming or pressure formingarticles and apparatuses, and, more particularly, a molding methodcombining vacuum and pressure for producing reinforced thermoplasticarticles. The invention also relates to molded articles havingreinforced foam fillers.

BACKGROUND OF THE INVENTION

Traditional blow molding is limited as to the wall thickness of thearticle to be formed, as well as the complexity of article shape. Toovercome this, thermoforming, a modification of blow molding, cansuffice for manufacturing articles having relatively thick walls and/orcomplex shapes. Thermoforming processes such as plug assisted vacuumforming or pressure forming permit the production of items having a wallthickness of up to about {fraction (3/18)} inch (95.25 mm). Articlesformed by conventional blow molding, by contrast, are usually limited towall thicknesses of less than about ⅛ inch (31.75 mm). This is due, inpart, to the negative effects exerted on the blowing process by thegreater volumes of polymer resin required to achieve thicker walls. Forexample, increasing amounts of viscous molten polymer will limit thesize, wall thickness and complexity of an article to be formed, as blownair becomes progressively ineffective at expanding molten polymer as thevolume of polymer material increases.

In basic vacuum forming, a carrier frame delivers a heated plastic sheetto a mold assembly, after which the sheet is clamped and sealed againstthe mold edge surfaces. Application of a vacuum causes atmosphericpressure to force the sheet against the mold cavity to assume the cavityshape. Mold cooling promotes the formation of a thin sheet having thedimensions defined by the mold.

As a variation of blow molding, the above-mentioned process furtherincludes the step of blowing air of controlled pressure to force theheated sheet away from the cavity into a bubble. A shaped plug is theninserted into the bubble, pressing the bubble back into the mold cavityafter the sheet has been sealed across the mold cavity. Upon reachingthe bottom of the mold cavity, compressed air and/or a vacuum is appliedto force the sheet against the mold. After forcing the sheet into thecavity, a full vacuum is applied from the cavity side and positivepressure is applied from the plug side of the apparatus to complete theformation of a molded article. After it has solidified, the moldassembly is opened, and the article is removed.

In a similar fashion, drape forming entails either draping a plasticsheet over or moving a male mold into a plastic sheet, and thereafterclamping, heating, and sealing the sheet over the male mold. Numerousvent holes in the mold apparatus permit a vacuum to be drawn, allowingatmospheric pressure to force the draped sheet into the contours of themold cavity. Upon cooling, the sheet shrinks onto the mold.

Typical vacuum-formed or pressure-formed products include blister andskin packaging, food and drink containers, toys, luggage, and auto andappliance parts. Polystyrene, polypropylene, HDPE, thermoplasticpolyester, ABS and vinyls are often used to manufacture these articles.Films and sheets formed in this fashion are often laminated by melt oradhesive processes to enhance their functional performance.

A need has arisen for reinforced blow molded articles having goodthermoinsulating and sound barrier properties. In particular, theresurgence in popularity of removable hard tops and T-tops forautomobiles has prompted engineers to seek better insulatingcharacteristics of blow molded articles. For example, lightweight,suitably thermoinsulated removable hard tops for sport utility vehicles(SUVs) are in high demand by consumers. While blow molding provides forsufficiently lightweight automobile parts, combining the suitable weightproperties with good impact resistance and thermoinsulating propertieshas heretofore been difficult.

The usefulness of blow molding techniques for forming such impactresistant, thermoinsulated articles has not been practical due to thestructural characteristics of the plastic material conventionally usedin blow molding. That is, the ability to blow mold light weight,thermoinsulated parts is limited by the fact that the parts produced canbe only so large or so thin before the parts lose their structuralintegrity and impact resistance.

Further, most insulating materials must be laminated to the part afterblow molding into the desired shaped. For example, urethane foam may beintroduced to a blow molded part to improve insulating capabilities, aswell as dimensional stability. However, this process is plagued byincompatibility between the skin component of the molded part and theinsulating foam filler. Expensive thermoplastic skins are oftenchemically incompatible with traditional foam insulating materials,preventing strong bond formation within laminated structures. Thus, blowmolded articles having skin and foam fillers of different materials areprone to delamination. A solution to the delamination problem is to fillthe article with a foamed resin identical to the resin used to form theexterior skin of the article. Although this expensive concept isacceptable for many blown articles, it is insufficient for producing acost-effective automobile part having good impact resistance.

Blow molded articles such as sport utility vehicle (SUV) hard topsrequire good thermoinsulation while exhibiting strong impact resistance.By nature, structural foams lack good impact resistance due to theiropen cellular conformation. Thus, blow molded automobile parts havingstructural foam insulating materials compatible with an exterior resinskin require reinforcement.

Heretofore, in order to reinforce various plastic parts, such partswould conventionally comprise resins fortified by mineral fillers orglass fibers. However, such reinforcement cannot be used effectively inblow molding operations because the glass fibers limit parison expansioncharacteristics and also have a deleterious effect on the blow moldingassembly itself. Furthermore, such reinforcement has a deterioratingeffect on the foaming capabilities of resins. Thus, blow molded articleshaving a structural foam component subjected to conventionalreinforcement often lack uniform strength and impact resistance.

Similarly, thermoformed articles having foam backing typically lacksatisfactory levels of impact resistance due to both the need for anaesthetically pleasing skin and the open cellular nature of reticulatedfoam. Exterior skin appearance deteriorates with increasing amounts ofconventional reinforcing materials. Typical reinforcing materials tendto impair the formation of reticulated cells during blowing of foamresins. Because structural foams are not adequately reinforced byconventional means, thermoformed articles comprising good quality skinslaminated to foam backing have inadequate strength and impactresistance.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the problems notedhereinabove. In achieving this object, the present invention provides amethod for thermoforming reinforced, insulated thermoplastic parts.Accordingly, the present invention provides a method for moldingarticles, comprising the steps of providing a first reinforced plasticsheet comprising at least one thermoplastic material and reinforcementnanoparticles dispersed within the at least one thermoplastic material.The reinforcement particles comprising less than 15% of a total volumeof the plastic sheet, and at least 50% of the reinforcement particleshaving a thickness of less than about 20 layers, and at least 99% of thereinforcement particles having a thickness of less than about 30 layers.The heated plastic sheet is communicated to a first mold assembly havinga first mold cavity defined by mold surfaces. The mold surfacescorrespond to a configuration of the article to be molded. An amount ofthe plastic sheet is communicated to the first mold assembly beingsufficient to form a skin of the article. A vacuum is applied to oneside of the first mold assembly while concurrently applying pressurizedgas to an opposing side of the first mold assembly so as to force theheated plastic sheet into conformity with the mold surfaces. Theconformed plastic sheet is then cooled. The conformed plastic sheet isthen transferred to a second mold assembly. A reinforced plastic meltmade from material identical or different from that of the plastic sheetis introduced to the conformed plastic sheet. The plastic melt has ablowing agent to achieve volume expansion and the production of acellular reticulate structure. The plastic melt is then cooled to form asolidified plastic member adhered to the conformed plastic sheet. Theconformed plastic sheet and the adhered solidified plastic membertogether comprise the article. The article is removed from the secondmold assembly.

It is also an object of the invention to produce reinforced parts forautomotive applications via plug assisted thermoforming, which hasheretofore been impractical.

An embodiment of the invention is a child safety seat having areinforced outer skin member and a reinforced foamed structural member.The seat members are formed from at least one thermoplastic material andreinforcement nanoparticles dispersed within the at least onethermoplastic material. The reinforcement particles comprise about 2% toabout 15% of a total volume of the molded hard top, at least 50% of thereinforcement particles have a thickness of less than about 20 layers,and at least 99% of the reinforcement particles have a thickness of lessthan about 30 layers.

In another embodiment, a substantially hollow molded hard top for anautomobile which is filled with foamed insulating material is formedfrom at least one thermoplastic material and reinforcement nanoparticlesdispersed within the at least one thermoplastic material. Thereinforcement particles comprise about 2% to about 15% of a total volumeof the molded hard top, at least 50% of the reinforcement particles havea thickness of less than about 20 layers, and at least 99% of thereinforcement particles have a thickness of less than about 30 layers.

Other objects and advantages of the present invention will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is described herein withreference to the drawing wherein:

FIG. 1 shows a perspective view of a sport utility vehicle hardtopcontemplated by the invention, and

FIG. 2 shows a sectional view of the top depicted in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is contemplated that reinforced skins according to the invention maybe prepared using any conventional pressure forming method. Preferably,the mold assembly is provided with appropriate water cooling lines and atemperature control unit in conventional fashion for regulating thetemperature of the mold assembly. The molds may assume a complex ordetailed shape, providing for reinforced complex shapes having areinforced foam core produced according to the invention.

In accordance with the present invention, the plastic melt (and thus theresultant part) comprises at least one thermoplastic material andreinforcement particles dispersed within the at least one thermoplasticmaterial. The reinforcement particles about 2% to about 15% of a totalvolume of the plastic melt, at least 50% of the reinforcement particleshave a thickness of less than about 20 layers, and at least 99% of thereinforcement particles have a thickness of less than about 30 layers.The reinforcement filler particles, also referred to as “nanoparticles”due to the magnitude of their dimensions, each comprise one or moregenerally flat platelets. Each platelet has a thickness of between0.7-1.2 nanometers. Generally, the average platelet thickness isapproximately 1 nanometer thick. The aspect ratio for each particle,which is the largest dimension divided by the thickness, is about 50 toabout 300.

The platelet particles or nanoparticles are derivable from largerlayered mineral particles. Any layered mineral capable of beingintercalated may be employed in the present invention. Layered silicateminerals are preferred. The layered silicate minerals that may beemployed include natural and artificial minerals. Non-limiting examplesof more preferred minerals include montmorillonite, vermiculite,hectorite, saponite, hydrotalcites, kanemite, sodium octosilicate,magadiite, and kenyaite. Mixed Mg and Al hydroxides may also be used.Among the most preferred minerals is montmorillonite.

To exfoliate the larger mineral particles into their constituent layers,different methods may be employed. For example, swellable layeredminerals, such as montmorillonite and saponite are known to intercalatewater to expand the inter layer distance of the layered mineral, therebyfacilitating exfoliation and dispersion of the layers uniformly inwater. Dispersion of layers in water is aided by mixing with high shear.The mineral particles may also be exfoliated by a shearing process inwhich the mineral particles are impregnated with water, then frozen, andthen dried. The freeze dried particles are then mixed into moltenpolymeric material and subjected to a high sheer mixing operation so asto peel individual platelets from multi-platelet particles and therebyreduce the particle sizes to the desired range.

The extruded plastic sheet utilized in accordance with the presentinvention is prepared by combining the platelet mineral with the desiredpolymer in the desired ratios. The components can be blended by generaltechniques known to those skilled in the art. For example, thecomponents can be blended and then melted in mixers or extruders.Preferably, the plastic melt is first manufactured into pellet form. Thepellets are then plasticized in the extruder to form a plastic melt,which exits the extruder in sheet form.

Additional specific preferred methods, for the purposes of the presentinvention, for forming a polymer composite having dispersed thereinexfoliated layered particles are disclosed in U.S. Pat. Nos. 5,717,000,5,747,560, 5,698,624, and WO 93/11190, each of which is herebyincorporated by reference. For additional background, the following arealso incorporated by reference: U.S. Pat. Nos. 4,739,007 and 5,652,284

Preferably, the thermoplastic used for the purposes of the presentinvention is a polyolefin or a blend of polyolefins. The preferredpolyolefin is at least one member selected from the group consisting ofpolypropylene, ethylene-propylene copolymers, thermoplastic olefins(TPOs), and thermoplastic polyolefin elastomers (TPEs).

The exfoliation of layered mineral particles into constituent layersneed not be complete in order to achieve the objects of the presentinvention. The present invention contemplates that at least 50% of theparticles should be less than about 20 nanometers in thickness and,thus, at least 50% of the particles should be less than about 20 layersthick. In addition, at least 99% of the reinforcement particles shouldhave a thickness of less than about 30 nanometers, which is about 30layers stacked in the thickness direction. With this extent ofexfoliation, with a loading of less than 15% by volume, the benefits ofthe nanoparticles begin to accrue with meaningful effect for many largethin part applications. For example, such loading of nanoparticles willprovide a desired increase in the modulus of elasticity by about 50-70%over conventional fillers. Preferably, about 2% to about 15%, even morepreferably about 2% to about 8% loading in used to achieve desirablereinforcement.

More preferably, at least 50% of the particles should have a thicknessof less than 10 nanometers. At this level, an additional increase ofabout 50-70% in the modulus of elasticity is achieved in comparison withthe 50% of particles being less than 20 layers thick as discussed above.This provides a level of reinforcement and impact resistance that wouldbe highly suitable for most motor vehicle part applications, such asreinforced insulated hard tops.

Preferably, at least 70% of the particles should have a thickness ofless than 5 layers, which would achieve an additional 50-70% increase inthe modulus of elasticity in comparison with the 50% of less than 10layer thickness exfoliation discussed above. This provides idealreinforcement and impact resistance for large thin parts that mustwithstand substantial impact. It is always preferable for at least 99%of the particles to have a thickness of less than about 30 layers, asparticles greater than this size act as stress concentrators.

It is most preferable to have as many particles as possible to be assmall as possible, ideally including only a single platelet.

As noted above, the preferred aspect ratio (which is the largestdimension divided by the thickness) for each particle is about 50 toabout 300. At least 80% of the particles should be within this range. Iftoo many particles have an aspect ratio above 300, the material becomestoo viscous for forming parts in an effective and efficient manner. Iftoo many particles have an aspect ratio of smaller than 50, the particlereinforcements will not provide the desired reinforcementcharacteristics. More preferably, the aspect ratio for each particle isbetween 100-200. Most preferably, at least 90% of the particles have anaspect ratio within the 100-200 range.

Generally, in accordance with the present invention, the plastic meltand hence the parts to be manufactured should contain less than 15% byvolume of the reinforcement particles of the type contemplated herein.The balance of the part is to comprise an appropriate thermoplasticmaterial and suitable additives. If greater than 15% by volume ofreinforcement filler is used, the viscosity of the composition becomestoo high and thus difficult to mold.

By utilizing plastic melt with the loading of nanoparticles discussedabove (e.g., less than 15% of a total volume of the plastic melt),higher modulus of elasticity of conventional large plastic parts can beachieved, and thus be manufactured with a reduced wall thickness whilemaintaining the same required impact resistance. For example, themodulus of the material used to form an article may be increased tobetween about 200,000 to about 500,000 PSI (1378-3446 MPa).

In accordance with the present invention, addition of the exfoliatedplatelet material as set forth above permits the modulus of vacuumformed articles to be increased without significantly losing impactresistance. Because the modulus is increased, large parts, such asremovable automobile hard tops, can be made thinner than what wasotherwise possible. Such parts may also be insulated by reinforced foam,thereby adding sound proofing and thermal insulation to thinner hardtops without jeopardizing impact resistance. More specifically, hardtops for automobiles must have sufficient impact resistance or toughnessto withstand various standard automotive impact tests, particularly rollover tests.

For example, an automotive hard top must withstand a typical impact testwherein the hard top will not crack or permanently deform upon impact.In a conventional IZOD impact test, it is desirable for the part towithstand at least 10-ft pounds/inch (535 J/m) at room temperature andat least 5-ft pounds/inch (263 J/m) at −300° C. In order to withstandcracking at such force levels, the modulus of a conventional automotivematerial is typically between about 70,000 to about 150,000 pounds persquare inch (PS) (482-1034 MPa). In accordance with the presentinvention, the hard top modulus can be increased by a factor of 2 to 3times, without significantly effecting the impact resistance.

In addition to the above mentioned benefits, use of the nanoparticlereinforced plastic melt enables the coefficient of linear thermalexpansion to be reduced to less than 40×10⁻⁶ inches of expansion perinch of material per degree Fahrenheit (IN/IN)/° F., or 72×10⁻⁶ mm/mm/°C., which is less than 60% of what was previously achievable forthermoplastic motor vehicle parts that meet the required impact tests.

As a further benefit, the surface toughness of the hard top can beimproved. The improved surface toughness provided by the nanoparticlesgreatly reduces handling damage and part scrap. This is a significantbenefit to a part which by design is repeatedly removed from anautomobile and must endure unexpected scraping, dropping andnon-collision impact.

In addition, it is possible to more than double the modulus of polymerswithout significantly reducing toughness. Thus, it is possible toproduce articles like hard tops using 20-35% thinner wall sections thatwill have comparable performance. The use of nanoparticles can providethe mechanical, thermal, and dimensional property enhancements, whichare typically obtained by adding 20-50% by weight of glass fibers ormineral fillers or combinations thereof to polymers. However, only a fewpercent of nanoparticles are required to obtain these propertyenhancements.

As a result of the fact that such low levels of nanoparticles arerequired to obtain the requisite mechanical properties, many of thetypical negative effects of the high loadings of conventionalreinforcements and fillers are avoided or significantly reduced. Theseadvantages include: lower specific gravity for a given level ofperformance, better surface appearance, toughness close to that of theunreinforced base polymer, and reduced anisotropy in the molded parts.

It is preferable for these articles to have reinforcement particles ofthe type described herein comprising about 2% to about 8% of the totalvolume of the article, with the balance comprising the thermoplasticsubstrate. It is even more preferable for removable hard tops to havereinforcement particles of the type contemplated herein comprising about3%-5% of the total volume of the part.

In accordance with another specific embodiment of the present invention,it is contemplated that the blow molding apparatus can be used to makerelatively large, highly reinforced parts having a modulus of elasticityof 1,000,000 (6892 MPa) or greater. Conventionally, these partstypically require loadings of 25-60% by volume of glass fiberreinforcement. This amount of glass fiber loading would result in a highviscosity of any melt pool that could be used in the blow moldingapparatus of the present invention and would thus render the blowmolding apparatus largely impractical for such application.

Sheets of the plastic melt described above enable the plug assistedthermoforming of large parts having impact resistance characteristicspreviously unattainable. For example, the thermoforming system of thepresent invention is able to manufacture relatively large articleshaving a modulus of elasticity of greater than 1,000,000 PSI (6892 MPa)by use of a plastic melt reinforced with loadings of about 8-15% byvolume of nanoparticles, with at least 70% of the nanoparticles having athickness of 10 layers or less. As with the above-described embodiment,the plastic melt used has substantially the same material composition asthe article to be manufactured.

in this case of molding large parts with a modulus of elasticity greaterthan 1,000,000 PSI (6892 MPa), it may be desirable to use engineeringresins instead of polyolefins. Such engineering resins may includepolycarbonate (PC), acrylonitrile butadiene styrene (ABS), a PC/ABSblend, polyethylene terephthalates (PET), polybutylene terephthalates(PBT), polyphenylene oxide (PPO), or the like. Generally, thesematerials in an unreinforced state have a modulus of elasticity of about300,000 PSI-350,000 PSI (2068-2412 MPa). At these higher loadings ofnanoparticles (8-15% by volume), impact resistance will be decreased,but to a much lower extent than by the addition of the conventional25-60% by volume of glass fibers.

The invention may be used to reinforce any item ordinarily produced bythermoforming. For example, removable automobile hard tops depicted inFIGS. 1 and 2, produced by plug assisted thermoforming may be reinforcedusing the inventive reinforcing particles. Such thermoformed hard topsfurther comprising structural foams having reinforcing nanoparticlesexhibit better impact resistance, thermoinsulation and sound insulationthan conventionally produced removable automobile hard tops.

Reinforced child safety seats may also be manufactured according to theinvention. Reinforcing nanoparticles of the invention can strengthen thethermoformed shell of the seat as well as the foam cushioning within theseat. Child seats reinforced with nanoparticles have better ductilityfor impact energy absorption than seats having standard reinforcingmaterials. The increased strength and impact resistance of such safetyseats affords better protection for seat occupants.

Reinforced articles having relatively thick walls may be producedaccording to the invention when the reinforced article comprises athermoformed skin blown from a reinforced polymer sheet under vacuumusing plug assistance. Larger, thicker, more complex articles may beformed according to the invention than is possible by blow moldingunreinforced polymers or polymers reinforced by, for example, glassfibers. This is because the reinforcing particles of the invention maybe evenly dispersed in molten resin, do not clump, and avoid generatingstress points likely to induce tears in the melted polymer during theblowing/forming step.

Although certain embodiments of the invention have been described andillustrated herein, it will be readily apparent to those of ordinaryskill in the art that a number of modifications and substitutions can bemade to the blow molding system disclosed and described herein withoutdeparting from the true spirit and scope of the invention.

What is claimed is:
 1. A method for thermoforming a reinforced article,comprising: providing a reinforced plastic sheet comprising at least onethermoplastic material and reinforcement nanoparticles dispersed withinthe at least one thermoplastic material, the reinforcement particlescomprising less than 15% of a total volume of the plastic sheet, atleast 50% of the reinforcement particles having a thickness of less thanabout 20 layers, and at least 99% of the reinforcement particles havinga thickness of less than about 30 layers; preheating said plastic sheet;communicating said preheated plastic sheet to a first mold assemblyhaving a first mold cavity defined by mold surfaces, the mold surfacescorresponding to a configuration of the article to be molded, an amountof the plastic sheet communicated to the first mold assembly beingsufficient to form a skin of the article; applying a vacuum to one sideof the first mold assembly while concurrently applying pressurized gasto an opposing side of the first mold assembly so as to force saidheated plastic sheet into conformity with the mold surfaces; cooling theconformed plastic sheet; transferring the conformed plastic sheet to asecond mold assembly; introducing to the conformed plastic sheet areinforced plastic melt made from material identical or different fromthat of the plastic sheet, said plastic melt having a blowing agent toachieve volume expansion and the production of a cellular reticulatestructure; cooling said plastic melt to form a solidified plastic memberadhered to said conformed plastic sheet, said conformed plastic sheetand said adhered solidified plastic member together comprising saidarticle; and removing said article from said second mold assembly.